The .303 is actually a better case for the heavier bullets than the .308.
Till they developed the Winchester Palma match case with semi balloonhead the short neck and short OAL of the .308 meant the base of heavier bullets could intrude into the powder space with resulting diminishing returns as powder charges had to be keep increasingly smaller in volume to avoid excessive chamber pressures.

Improved propellents have offset that diminishing return aspect. Still with the heavier bullets chamber pressures are greatly increased compared to the .303 with relatively little increase in velocity.